Three-dimensional data fields are quite common in many applications, and visualization of 3D data fields is a hot research area in scientific visualization. Among all the visualization methods, direct rendering to 3D data is a fundamental and important approach. In comparison with the surface extraction approach, direct volume rendering processes all samples of data in making images, without necessity of producing the intermediate geometric elements. Therefore, more features inside data could be demonstrated in a single image, and more information of the data could be provided to investigators. Although the direct volume rendering has many advantages, the method is computationally extensive and the images produced by the method are in general blurred, which hinders scientists from understanding data in high efficiency. To solve the problems. several aspects are investigated and research on the problem solution has been conducted in the works associated with the thesis. 1) By establishment of a transparent degree unit to unify different transparent values of samples, a transparent degree ruler is set up to facilitate the transparency accumulation Based on such a ruler, the operation of color composition, the fundamental calculation in the volume rendering can be greatly simplified, through a replacement of summation operations provided in the method in stead of the original multiplication operations. 2) By distributing the accumulation colors along a ray over a 2D virtual plane, much 3D information of the data volume could be reflected on the plane. Through the color distribution in this novel proposal, a new interactive visualization method is put forward which allows the interactions being operated on the virtual plane. With the new interactive method, an investigator has the power of manipulating parts of data at his/her disposal which he/she is interested in. 3) As a special projection method in volume rendering, splatting works well for regular data volumes. However, the method performs poorly when irregular volumes are rendered. By using the triangular Sine function to uniformly scale samples in different sizes, a variable splatting algorithm is proposed to have different samples accumulate their effects in high precision. The method may greatly improve the image quality. 4) Shading is an important measure to help the expression of data features in visualization. However, it is difficult to produce shading effect in the projection approach. In this thesis, we present a shading technique by using sphere and ellipsoid surfaces to approximate the material interfaces in data volumes. As a result, shading effect may be easily performed on the material interfaces in the projection approach. 5) In irregular data volumes, as the shape and size of samples are not constant, the rendering speed is rather low and the quality of images produced are also far form satisfaction. In this theis, we put forward a method to solve the problem by setting up node links in queuing up cells at each pixel. Consequently, three-dimensional scan conversion & ray-casting are naturally combined, and the rendering process could be performed with high efficiency of projection approach and the image quality of ray-casting approach. 6) Rendering volume data with geometric objects together is an important subject in scientific visualization. Although many methods have been proposed, the problem of intersection between geometric objects and volume samples has not been completely solved yet, and no one has employed the projection approach in this aspect. After an investigation of the intersection situation, we present a method by setting up node links to integrate the projection approach. The method completely solves the intersection problem with high efficiency in rendering the geometric objects and volumes together.